Apolipoprotein (apo) A-IV is a 46 Kd glycoprotein that is synthesized by the intestinal enterocytes of mammals during lipid absorption and secreted into lymph on the surface of chylomicrons. Although a broad spectrum of physiologic functions has been proposed for apo A-IV, a preponderance of evidence suggests that its primary role is in intestinal lipid absorption. Apo A-IV has a low interfacial exclusion pressure, and thus its binding to lipoproteins is sensitive to the physical state of their surface. We have proposed that this behavior enables apo A-IV to act as a barostat that maintains lipoprotein surface tension and lipid packing within a critical range required for lipid transfer reactions. Using novel surface chemistry techniques, we have found that apo A-IV displays unique elasticity at expanding interfaces, and that C-terminal mutations in apo A-IV alter its interfacial activity, elasticity, and ability to modulate lipid exchange reactions. Preliminary studies in COS, McA-RH7777, and IPEC cells suggest that apo A-IV interacts with apoB during triglyceride-rich particle assembly and alters lipoprotein trafficking and secretion. Thus, we propose that the unique dynamic interfacial properties of apo A-IV play a specific role in triglyceride-rich particle assembly, specifically, that apo A-IV acts in the endoplasmic reticulum and Golgi to modulate interfacial tension and lipid packing at the nascent chylomicron surface, thereby controlling lipidation and expansion, intracellular trafficking and secretion kinetics and, ultimately, the efficiency of intestinal lipid absorption. To pursue this hypothesis we propose three specific aims: 1) we will study informative apo A-IV site-directed mutants using spectroscopic and surface chemistry techniques to elucidate the structural determinants of its interfacial elasticity and interaction with lipid and apo B;2) we will transfect COS, McA-RH7777, and IPEC cells with biophysically characterized apo A-IV mutants to examine how the interfacial behavior of apo A-IV modulates assembly, trafficking, and secretion of triglyceride-rich lipoproteins;3) we will use apo A-IV knockout and mutant apo A-IV transgenic mice to examine the effect of intestinal apo A-IV expression on the efficiency of fatty acid absorption and the size and composition of intestinal lipoproteins. We believe that these studies will provide new knowledge on the function of apo A-IV in the complex process of lipoprotein assembly relevant to the dietary control of atherosclerotic cardiovascular disease and obesity.